A semiconductor laser is ordinarily made of Group III-V semiconductor materials. One particularly useful form of such a laser has distributed feedback ("DFB"). That is to say, optical feedback is built into the laser along its cavity length. For example, such feedback is supplied by means of a DFB diffraction grating whose stripes ("teeth") run perpendicular to the length (longitudinal direction) of the laser cavity. Such lasers, however, tend to suffer from spatial holeburning (spatial variation in optical gain saturation along the longitudinal direction) and from adiabatic chirping. That is to say, they suffer from relatively low gain near the highly reflecting mirror of the laser owing to spatial variation in gain saturation, and from non-symmetrical spectral intensity distribution around the spectral maximum. In turn, such holeburning and chirping cause, among other things, an undesired lack of single mode operation as well as an undesired lack of linearity of laser response to applied signals.
U.S. Pat. No. 5,329,542 teaches a semiconductor DFB laser in which improved single-mode behavior of such a laser is improved by reducing the feedback at or near one or both ends of the DFB grating. In particular, the feedback is reduced by reducing the coupling coefficient .kappa. between the DFB grating and the optical cavity. This reducing in the coupling coefficient .kappa. is achieved by spatially varying the depth of the grating's teeth and/or by spatially varying the spacing between adjacent teeth (while maintaining a fixed periodicity).
The aforementioned patent further teaches methods to achieve this spatially varying coupling .kappa.. More specifically, the patent teaches a method of spatially varying either the depths or the spacing of the teeth. The method uses a crystallographically dependent (angular; non-vertical) chemical etching in combination with a patterned photoresist masking layer having a spatial duty cycle (i.e., variable tooth density) that varies in the longitudinal laser direction. This spatially varying duty cycle cannot be achieved by a relatively quick method--for example, a holographic interference exposure method--that produces a constant duty cycle over the entire surface in a single exposure; but it requires other, more time consuming methods. The resulting tooth depth is a critical parameter that determines .kappa.. Such a method thus critically relies on the chemistry of the crystallographic etching required to produce the teeth. Consequently the angle of etching, and hence the depth of the teeth, is very sensitive to the chemistry of the crystallographic etching. Therefore it is relatively difficult to control the average coupling resulting in the gratings made by such methods. It would therefore be desirable to have a method of making a DFB laser with a coupling .kappa. that varies in the longitudinal laser direction but which does not rely on crystallographic etching and which can be achieved with a constant spatial duty cycle.